My stutter is pretty intermittent so I usually wait and see if I actually stutter, and then pause to briefly explain (e.g., "Excuse me, I have a speech impairment" \[pause, breathe, restart problematic sentence\]). But that's because I have a weird thing where I can be stuck on a particular word or sentence, then go to say something else and that will be completely fluent (but the problematic word/sentence remains problematic). So this approach doesn't work for everyone.